Self-assembly of amphiphiles, polymers and proteins at interfaces between thermotropic liquid crystals and aqueous phases

We demonstrate a variety of ordered patterns, including hexagonal structures and chains, formed by colloidal particles (droplets) at the free surface of a nematic liquid crystal (LC). The surface placement introduces a new type of particle interaction as compared to particles entirely in the LC bulk. Namely, director deformations caused by the particles lead to distortions of the interface and thus to capillary attraction. The elastic-capillary coupling is strong enough to remain relevant even at the micron-scale when its buoyancy-capillary counterpart becomes irrelevant.

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Ordered droplet structures at the liquid crystal surface and elastic-capillary colloidal interactions.

Through the combination of three different, complementary techniques (optical microscopy, x-ray diffraction and atomic force microscopy), we reveal the deformations inside thin smectic films frustrated between two interfaces imposing antagonistic anchorings. We study the model system, 4-n-octyl-4'-cyanobiphenyl (8CB) between MoS2 and air, which is characterized by the competition between homeotropic anchoring at air and planar unidirectional anchoring on the substrate, with thicknesses varying around 0.3 microm. Optical microscopy and x-ray diffraction demonstrate the continuous topology of smectic layers between the interfaces, which are stacked into periodic flattened hemicylinders. These latter are one-dimensional (1D) focal conic domains which form an optical grating in the smectic film, of a period ranging from 1 to 2.5 microm. The interpretation of our results through an energetic model, associated with the atomic force microscopy (AFM) measurements, shows the presence below a critical thickness of a new type of curvature wall between neighboring hemicylinders.

Optical gratings formed in thin smectic films frustrated on a single crystalline substrate.

The liquid crystal dimer 1ʺ,7ʺ-bis(4-cyanobiphenyl-4ʹ-yl) heptane (CB7CB) has been shown to possess a nematic–nematic phase transition at 376 K. The phase below this temperature has been identified...

Twist-bend nematic phase of the liquid crystal dimer CB7CB: orientational order and conical angle determined by 129Xe and 2H NMR spectroscopy

Cholesteric blue phases are liquid crystalline phases in which the constituent rod-like molecules spontaneously form three-dimensional, helical structures. Despite theoretical predictions that they are composed of cylindrical substructures within which the liquid crystal molecules are doubly twisted, real space observation of the arrangement of such structures had not been performed. Through transmission electron microscopy of photopolymerized blue phases with controlled lattice plane orientations, we report real space observation and comparison of the lattice structures of blue phases I and II. The two systems show distinctly different contrasts, reflecting the theoretically predicted, body centred and simple cubic arrangement of the double-twist cylinders. Transmission electron microscopy also reveals different tendencies of the two blue phases to align on unidirectionally rubbed surfaces. We thus show that TEM observation of alignment-controlled, photopolymerized liquid crystals can be a powerful tool to investigate complex liquid crystalline order.

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Double-twist cylinders in liquid crystalline cholesteric blue phases observed by transmission electron microscopy.

A composite prepared from aerosil A380 and the liquid crystal (LC) 4-hexyl-4′-cyanophenyl benzoate (CP6B) was investigated by broadband dielectric spectroscopy in a large temperature range. The selected high silica density (ca. 7 g aerosil/1 g of CP6B) allows the observation of a thin layer (two-monolayer structure) adsorbed on the surface of the silica particles. For the composite one relaxation process is observed at frequencies much lower than that of the processes found for bulk CP6B. It is assigned to the dynamics of the molecules in the surface layer. The temperature dependence of its relaxation rates obeys the Vogel–Fulcher–Tammann law, which is characteristic for glass-forming liquids. The quasi 2D character of the observed glassy dynamics in the surface layer is discussed. The temperature dependence of the CP6B relaxation in the composite is compared with that of related hexylcyanobiphenyl molecules in the surface layer of aerosil composite with a similar concentration.

Dynamics of Cyanophenyl Alkylbenzoate Molecules in a Surface Layer Adsorbed onto Aerosil. I. Cyanophenyl Hexylbenzoate

Temperature induced surface transitions in liquid crystal tilted homogeneous cells are experimentally studied. The results are theoretically explained by an elastic theory of surface transitions and the resulting values for the effective splay-bend elastic constant k 13 for each liquid crystal material are compared with the results from experience with other types of surface anchoring.

Temperature induced surface transitions in liquid crystal tilted homogeneous cells

Orientational properties of rubbed thin polymer layers with vinylic backbone and different side groups are presented. Some of these polymers lead to the alignment of LC molecules parallel to the rubbing direction whereas other polymers impose a perpendicular orientation. The stability of the obtained alignment as function of the temperature was tested.

Alignment Properties of Some Polymeric Layers

In the paper, the physico-chemical orientation mechanism on solid surfaces for a liquid crystal (NP5 from Merck) doped for homeotropic alignment, is tested. For that, the dispersion and polar parts of surface tension of the liquid crystal are determined. Using substrates with and without adsorbed surfactant, the dispersion and polar parts of the surface tension of the substrates were also measured. Comparison of liquid and solid surface tensions, involved in the physico-chemical mechanism, failed to explain the difference of the molecular alignment for the two substrates. In the presence of the surfactant adsorbed on the surface, the steric model probably gives the explanation for the homeotropic alignment.

Mechanism of homeotropic alignment of a doped liquid crystal

Alignment layers containing copper(II) complexes with acrylic acid or polyvinylpyrrolidone show homeotropic orientational properties. This behaviour is supposed to be due to the particular structure of the complex copper(II) ions. We give spectroscopic arguments to support the anchoring of liquid crystal (LC) molecules by coordination to the copper ion within the alignment layer.



Des couches contenant des complexes de cuivre(II) aux ligands organiques comme l'acide acrylique et le polyvinylepyrrolidone montrent des proprietes d'orientation homeotrope. On peut supposer que ces proprietes sont liees a la structure particuliere du complexe de cuivre. Des arguments spectroscopiques soutiennent l'idee de l'encrage par coordination des molecules du cristal liquide a l'ion du cuivre apartenant a la couche d'orientation.

R. Moldovan

Papers

Dynamics of Cyanophenyl Alkylbenzoate Molecules in a Surface Layer Adsorbed onto Aerosil. I. Cyanophenyl Hexylbenzoate

Temperature induced surface transitions in liquid crystal tilted homogeneous cells

Alignment Properties of Some Polymeric Layers

Mechanism of homeotropic alignment of a doped liquid crystal

On Some Transition Metal Complexes Having Orientational Properties(II). Copper(II) complexes